The invention pertains to thermomechanical optical sensors and in particular to optical sensors adapted to independently sense temperature and strain.
General State of the Art
Various optical sensors for measuring temperature and strain are known in the art. One such sensor, known as the in-line fiber etalon (ILFE) sensor, is sensitive to strain in the fiber axis direction but is insensitive to temperature and strain components that are not in the fiber axis direction. The ILFE sensor is made by fusing two cleaved single mode fibers to a same diameter capillary tube forming an air filled cavity of a given length. When the sensor is illuminated from one end, two Fresnel reflections are created at the glass/air interfaces at the cleaved fiber ends. These reflections propagate back toward the light source, where they are directed by an optical phase detection system. Because the second reflection travels an extra difference equal to the round trip distance through the air cavity, it is out of phase with respect to the first reflection. This phase difference can be measured to determine the length of the air cavity. When such a device is bonded or imbedded into a structure, any elongation of the structure results in an elongation of the sensor cavity. Thus such a device may be used to monitor the strain by measuring the length of the sensor air cavity. However, the device is temperature sensitive, and temperature compensation is required to obtain accurate results.
One known optical phase detection system for ILFE sensors is to use path match differential interferometry (PMDI). Such systems use a low coherence light source such as a light-emitting diode to illuminate the sensor. Light returned from the sensor is directed to an unbalanced reference interferometer which is located in a readout system. In such a configuration if a difference in length between two legs of the reference interferometer is substantially equal to the round trip distance for the sensor cavity, then the signal is reconstructed at the output for the reference interferometer. As the offset of the reference interferometer is adjusted over a large range, the sensor signal will vary in strength from zero to a maximum and then back to zero. The sensor signal is maximum when the offset of the reference interferometer is equal to the round trip distance through the sensor cavity. By measuring the sensor signal level as a reference interferometer is tuned, an absolute measurement of the sensor gap length can be made. As a result, the strain is also known because the air gap length is uniquely related to the sensor strain level.
It would be useful to employ a sensor which can measure temperature in addition to strain. However, it is of little use to have a device that is sensitive to both strain and temperature if only the combination of the two effects is known. Past attempts to provide sensors for measuring both temperature and strain have used either a combination of interferometric and polarimetric sensors, two interferometers along independent Eigen axes or two Bragg gratings in a high birefringent optical fiber. These approaches are generally prone to high errors and instabilities because the equations that must be solved are not sufficiently independent. Such equations are known, and examples of the equations are disclosed in the provisional application noted above.
It is therefore desirable to develop a sensor capable of measuring axial strain and temperature on the surface or inside of a structural member.